1. Field of the Invention
The present invention relates to a location registration and paging procedure for mobile stations included in a mobile communication system and, more particularly, to a location registration and paging procedure for mobile stations each being movable in a plurality of radio zones in a mobile communication system which has overlapping location areas.
2. Description of the Prior Art
A conventional car telephone, portable telephone or similar mobile communication system is implemented with a miniature zone scheme in which the entire service area belonging to the system is covered by a plurality of radio zones. The term "radio zone" refers to a range in which each radio base station is communicable with a mobile station mounted on a car, for example, over a radio channel. A single radio base station is situated in each radio zone, and a plurality of mobile stations each is freely movable in the radio zones.
In a mobile communication system of the type using miniature zones as mentioned above, each mobile station has to register the location thereof to inform a radio base station of the current radio zone in which it exists. To avoid an increase in the traffic of exclusive control signals for location registration, it has been customary to divide the entire service area into sets of radio zones which neighbor each other to thereby define a plurality of location areas. A mobile station registers the location every time it is handed over from one location area to another. When a terminating call meant for a certain mobile station arrives, radio base stations situated in the location area where the mobile station has registered the location call the mobile station by broadcasting. Each location area is provided with an adequate size in consideration of the fact that including an excessively great number of radio zones therein would increase the calling traffic and thereby reduce the efficiency.
The way to divide the entire service area into a plurality of location registration areas may generally be classified into two kinds, i.e., one for non-overlapping location registration areas in which a radio zone belongs to a fixed location registration area and the other for overlapping location areas in which a radio zone belongs to a plurality of location registration areas.
Regarding the number, the non-overlapping location areas each has a relation of 1: n (natural number) to the radio zones belonging thereto, i.e., a radio zone is prevented from belonging to a plurality of location areas at the same time. In a mobile communication system, it sometimes occurs that a mobile station positioned in the vicinity of the border between nearby location areas moves across the border repetitively. In such a case, the non-overlapping location areas would result in the increase in the frequency of location registration of the mobile station and thereby cause the control signal traffic for location registration to center on radio base stations situated in the vicinity of the border.
The overlapping location areas eliminate the above problem. Specifically, if the duplication of radio zones in a first and a second location area is admitted, a mobile station does not have to register the location repetitively so long as it moves in the radio zones belonging to both of the first and second location areas. This is successful in preventing the control signal traffic from centering on particular radio base stations. To set up an overlapping location area, a set of a plurality of radio zones neighboring the lastest radio zone where a mobile station has registered the location may be defined as a second location area.
A specific method of setting up overlapping location areas was reported at THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, JAPAN, 1989 SPRING NATIONAL CONVENTION, Paper No. B-863. This method sets up a location area on a radio zone basis by using two-dimensional positional coordinates particular to each radio base station.
Specifically, the above-mentioned method is such that when a mobile station has registered the location in a radio base station belonging to a given radio zone, it memorizes positional coordinates being notified by the radio base station. Every time the mobile station is handed over from one radio zone to another, it receives positional coordinates being notified by the radio base station situated in the new radio zone. When the mobile station enters a radio zone whose radio base station is situated at a predetermined distance R from the radio base station in which the mobile station has registered the location previously, it registers the location again.
However, the location area based on the coordinates of the radio base station in which a mobile station has registered the location has fixed topography with no regard to the position of the mobile station. This brings about a problem that even when a mobile station moves only one-dimensionally such as on a straight thruway, a location area including radio zones which the mobile station will not pass is set up.